1. Field of the Invention
This invention relates to an optical communications system, and to terminal equipment forming part of such a system. More particularly, this invention relates to an optical communications system comprising a plurality of terminals, interconnected by a light path (particularly but not exclusively, an optical fiber cable), and arranged to signal using selected wavelengths from a plurality of possible simultaneous signalling wavelengths.
2. Related Art
Optical communications systems are suitable for applications in which large volumes of data are to be communicated between stations, such as local area networks (LANs), or Metropolitan area networks (MANs). It is known to provide such networks using optical fiber cable to interconnect network stations, and signalling on selected ones of a plurality of wavelengths using wavelength division multiplexing (WDM).
One example of such a system, and specifically a terminal station for such a system, is described in the Proceedings of the Nineteenth European Conference on optical communication (ECOC' 93) Volume 2, paper TuP4.4, pages 121-124, Sep. 12-16 1993, I Chlamtac et al "A Multi-Gbit/s WDM Optical Packet Network with Physical Ring Topology and Multi-subcarrier Header Encoding". In the system there described, each terminal station consists of a laser tuned to operate at a single discrete frequency (different to the frequencies of all other lasers of all other stations in the system), a subcarrier receiver tuned to operate at a single discrete subcarrier frequency (different to the frequencies of all other receivers of all other stations in the system), and a tunable wavelength selector capable of selectively tuning to any of the transmitter wavelengths. All the stations are connected by a single optical fiber cable. Data is communicated in packets, all having the same predetermined length. When a station wishes to transmit a packet, it transmits a header on the subcarrier of the station to which the packet is to be sent, and then sends the data on its transmit wavelength by using its laser diode, the output of which is then coupled to the fiber. At the destination station, the header on a subcarrier is detected. The header includes an indication of the transmitting station, and therefore the transmitting wavelength, and this is used to tune the wavelength selector to the correct receiving wavelength, and the packet is received ("dropped") via the wavelength selector.
Because of the high bandwidth of optical fibers (or optical paths in general), it is possible to provide a reasonably large number of stations, each operating at a high data rate using this type of wavelength multiplexing system.
Further, because every station has its own transmit wavelength, there is no possibility of collision between data from different stations on the same wavelength.
However, the system does have several drawbacks. Firstly, it requires every station to have a different transmitting frequency, and this means either manufacturing a very large number of fixed frequency laser diodes of different frequencies, or providing a tunable laser at every station (which would require accurate wavelength stabilisation equipment at each station to avoid cross-talk between wavelengths). The same applies to the need for a separate subcarrier receiver for each station. Finally, the total number of stations must inevitably be limited to the total number of available wavelengths (and/or subcarriers).
EP 0452895 discloses an optical network system which comprises a plurality of terminal stations interconnected by an optical fiber cable. In a first embodiment, a base station transmits a plurality of different wavelengths. A first wavelength variable filter continually scans all the wavelengths, to attempt to find a free wavelength. When a free wavelength is found, the current setting of the first wavelength variable filter is used to set a second wavelength variable filter which extracts the free wavelength. The extracted free wavelength is modulated by an optical modulator, and recombined with the other wavelengths in a multiplexer. The initial part of the data transmitted by the modulator is an indication of the destination station for the data. All stations, therefore, also scan all the wavelengths to attempt to locate such a header indicating that data is addressed to them. When such a header is located, the second wavelength variable filter is set to the wavelength on which the header occurred, and the subsequent data is demodulated using a photosensor.
In the second embodiment, the method of reception of data is as in the first embodiment. The method of transmission of the data from a station differs, however, in that, instead of using an optical modulator to modulate the extracted free wavelength, two laser diodes are employed to generate free wavelengths which are modulated by optical modulators and multiplexer into the signals on the optical fiber. As before, a wavelength variable filter sweeps the available wavelengths to search for a free wavelength, and the laser diodes are set to the or each free wavelength. The laser diodes are stabilised by the transmission, from the base station, of a reference wavelength which is extracted by a separate wavelength variable filter, and used to control the laser diodes at each station.
Both embodiments thus avoid the need for every station to have a different transmitting frequency and a different receiving frequency, and hence either large numbers of laser diodes or temperature stabilisation at each station (although, in the second embodiment, some wavelength stabilisation circuitry is needed).
However, this is achieved only by sacrificing a major advantage of the first described (Chlamtac) system above; namely, its immunity from collision. In the system of EP 0452895, collision is highly likely because all stations are simultaneously scanning the free wavelengths in order to be able to transmit data. Thus, several stations may simultaneously detect that the same wavelength is free, and attempt to transmit data at the same time. Obviously, in this instance, all the transmitted data on that wavelength will become corrupted. For this purpose, EP 0452895 proposes to use the transmission protocol known as carrier sense multiple access/collision detection (CSMA/CD), in which stations detect collision of data and attempt re-transmission. However, this in turn can lead to repeated collisions, as the re-transmissions themselves collide; and, in any case, leads to delay in the transmission of data, and the need for further complicated circuitry to deal with the control of the collision protocols.
Furthermore, because each station needs continually to scan all frequencies to determine the wavelength on which data for that station may be transmitted, the rate of transmission is limited by the rate of scanning of the wavelength variable filter and the number of wavelengths to be scanned; since, if a destination indicating header is only scanned part way through, the receiving station may not correctly decode the destination, and accordingly may not decode the signal. For this reason, some form of acknowledgment signalling, and associated re-transmission of data, would appear to be increasingly necessary as the speed of transmission or the number of wavelengths employed in this system increases.